A gas turbine is a combustion engine which comprises a turbine with a compressor connected upstream and a combustion chamber connected between them. In this, air is first compressed via the blading of one or more compressor stages, then in the combustion chamber is mixed with a gaseous or liquid fuel, ignited and combusted. In addition, the air is used for cooling. In this way a hot gas (mixture consisting of combustion gas and air) results, which is expanded in the subsequent turbine section, wherein thermal energy is converted into mechanical energy. This mechanical energy first drives the compressor, the remaining portion being used for example for driving a generator.
The compressor customarily comprises a plurality of rotor wheels, with compressor blades, in an axial type of construction. It converts the kinetic energy of the inflowing air mass in the diffuser-like, i.e. diverging, interspaces of the compressor blades into pressure energy. The kinetic energy which is lost in the process is recompensated in a rotor stage. A complete compressor stage of an axial compressor therefore comprises a rotor stage, in which both pressure and temperature as well as speed increase, and a stator stage, in which the pressure rises to the disadvantage of speed. The rotor stages are arranged one behind the other on a number of drums, and the stator stages are built into the inner side of the compressor casing in a fixed manner.
The high compression of the air creates a sharp temperature rise. The air which is heated in this way then flows into the combustion chamber where a fuel is fed to it. During engine start, igniter plugs ignite the fuel, then combustion is carried out continuously. As a result of the combustion, the temperature rises again and the gas expands.
The gases which flow from the combustion chamber then impinge upon a turbine where their kinetic and thermal energy is converted into mechanical energy. Via a shaft, this first drives the compressor, and, depending upon the design purpose of the gas turbine, drives a generator for power generation.
Gas turbines are used today in gas and steam turbine plants (GuD plants) and serve there predominantly for power generation. In this case, a modern GuD plant customarily comprises one to four gas turbines and at least one steam turbine, wherein either each turbine drives a generator in each case (multishaft installation), or one gas turbine with the steam turbine on a common shaft drives a single generator (single-shaft installation). The hot exhaust gases of the gas turbine in this case are used in a heat recovery steam generator for producing steam. The steam is then fed to the steam turbine. Customarily, the gas turbine accounts for about ⅔ of the electric power and the steam process accounts for about ⅓.
Depending upon availability of the source of energy, a GuD power generating plant can also be designed as an IGCC (Integrated Gasification Combined Cycle) plant. In this case, a fuel gasification is connected upstream to the GuD process. Primary energy (coal, biomass, waste) is gasified in this case in a gasifier, forming an energy-rich gas. The resulting crude gas is cooled, purified and in the process passes through desulphurizing plants, filters and other units. The synthesis gas which is produced in this way is then fed to the gas turbines of the GuD plant.
For producing the synthesis gas, depending upon concept, air is extracted at the compressor end of the gas turbine and separated in an air-separation plant into its main constituents of oxygen and nitrogen. In the fully integrated IGCC operation, air which is extracted exclusively from the compressor end of the gas turbine is used in this case for air separation, and in the case of the partially-integrated concept an additional external compressor is provided. The oxygen which is produced in the air-separation plant is used for synthesis gas production, and some of the nitrogen which accumulates in the air-separation plant as a byproduct is admixed with the synthesis gas and combusted in the combustion chambers of the gas turbine.
The controlling of the gas turbine is customarily carried out by means of load and temperature controllers. In so doing, the load controller customarily undertakes the maintaining/adjusting of the load reference value via adjustment of the fuel valve, and the temperature controller customarily undertakes the maintaining of a specified turbine exhaust temperature by means of adjustment of the compressor inlet guide vanes, i.e. of the guide vanes at the inlet of the compressor.
Furthermore, from US 2008/0047 275 A1 it is known to control the pressure at the exit of the compressor of a separately operated gas turbine.
Particularly in the case of the IGCC processes which are described above, instabilities (especially in the adjustment range of the inlet guide vanes) between the turbine exhaust temperature controlling and the air-separation plant can occur in the case of the customary control concept. Already slight alterations of the inlet guide vane position of the compressor of the gas turbine in this case can lead to severe fluctuations of the volume of extracted air which then react again upon the gas turbine and in the manner of a cascade lead to fluctuations which can no longer be corrected. Such severe instabilities can only be prevented by blocking of the inlet guide vanes by manual intervention of the operating personnel, wherein, however, the load controlling of the gas turbine is no longer possible. The fluctuations can even be so severe that the availability of the gas turbine and adjacent systems (such as the air-separation plant) can possibly no longer be ensured.